Chapter: Haloalkanes and Haloarenes Characteristics of halo compounds. Question 1 Write the IUPAC name of the following compound:(CH3)3CCH2Br Ans.

The IUPAC name of the given structure is 2, 2-Dimethylbromopropane. Question 2

Ans.

Question 3 Give the common name of the following. (i) 1- Bromopropane (ii) 2-chloro propane (iii) 1-chloro-2-methylpropane Ans. (i) Common name of 1- Bromopropane is n-propylbromide (ii) Common name of 2-chloro propane is Isopropyl chloride (iii) Common name of 1-chloro-2-methylpropane Isobutyl chloride

Question 4 Classify the following as alkyl, allyl, benzyl, vinyl or aryl halides: (i) (CH3)2CHCH(Cl)CH3 (ii) ( CH3)3CCH2CH(Br)C6H5 (iii) CH3CH=C(Cl)CH2CH(CH3)2 (iv) CH3CH=CHC(Br)(CH3)2 (v) p-ClC6H4CH2CH(CH3)2 Ans. (i) Alkyl halide: (CH3)2CHCH (Cl) CH3 (ii) Allyl halide: CH3CH=CHC (Br) (CH3)2 (iii) Benzyl halide: (CH3)3CCH2CH (Br) C6H5 (iv) Vinyl halide: CH3CH=C (Cl) CH2CH (CH3)2 (v) Aryl halide: p-ClC6H4CH2CH (CH3)2 Question 5 Write the isomers of the compound having formula C4H9Br. Ans. i. CH3CH2CH2 CH2Br ii. CH3CH2 CH (Br)CH3 iii. CH3CH(CH3) CH2Br iv. CH3CBr(CH3) CH3 Question 6 Define racemisation? Ans. A mixture containing two enantiomers in equal proportions is known as racemic mixture and the process of conversion of enantiomers into a racemic mixture is known as racemisation. Question 7 What are enantiomers? Give example. Ans. The stereoisomers related to each other as nonsuperimposable mirror images are called enantiomers. For example – 2-chloro butane (CH3CH2 C*H (Cl) CH3). Here the carbon atom marked is the chiral centre and posses four different group.

Question 8 What is benzylic halide? Give the structure. Ans. The compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom next to an aromatic ring is called benzylic halide.

Question 9 What is the common name of 1, 3-Dibromobenzene? Ans. m- Dibromobenzene. Question 10 What is the general formula for alkyl halide? Ans. CnH2n + 1X, where X is halogen atom and n is the number of carbon atom. Question 11 Write the structures of the following organic halogen compound. 2-Chloro-3-methylpentane. Ans. CH3CH (Cl) CH (CH3) CH2CH3 Question 12 Give the IUPAC name of the compound CH3CH (Cl) CH (Br) CH3. Ans. 2-bromo-3-chloro-Butane

General methods of preparation Question 1 Write the structure of the major organic product in each of the following reactions: (i) CH3CH2CH2OH + SOCl2

(ii) CH3CH2CH = CH2 + HBr (iii) CH3CH = C (CH3)2 + HBr

Ans. (i) CH3CH2CH2Cl (ii) CH3CH2CH2CH2Br (iii) CH3CH2 CBr(CH3)2

Question 2 Write the equations for the preparation of 1-iodobutane from (i) 1-butanol (ii) 1-chlorobutane (iii) But-1-ene. Ans.

(i) CH3CH2CH2CH2OH CH3CH2CH2CH2I 1-butanol 1-iodobutane (ii) CH3CH2CH2CH2Cl + NaI CH3CH2CH2CH2I + NaCl 1-chlorobutane 1-iodobutane (iii) CH3CH2CH = CH2 + HI CH3CH2CH2CH2I But-1-ene. 1-iodobutane Question 3 Complete the following reactions: (i)

(ii)

(iii) CH3CH2Br + NaI Ans. (i)

+ SO2 + HCl (ii)

(iii) CH3CH2Br + NaI CH3CH2I + NaBr Question 4 p-Dichlorobenzene has higher melting point and solubility than those of ortho- and meta-isomers. Discuss. Ans. Due to its symmetry p-Dichlorobenzene fits in crystal lattice better as compared to o- and m-isomers. As a result the melting point and solubility of p-Dichlorobenzene is higher than ortho- and meta-isomers. Question 5 Explain the statement "Alkyl halides, though polar, are immiscible with water". Ans. Energy is required to overcome the attractions between the haloalkane molecules and to break the hydrogen bonds between water molecules. The energy released during the formation of new attractive force between Haloalkane and the water molecule is very less as the attractions between the haloalkane and the water molecule is not as strong as the original hydrogen bonds in water. Thus Alkyl halides, though polar, are immiscible with water.

Question 6 Why is sulphuric acid not used during the reaction of alcohols with KI? Ans. H2SO4 converts KI to corresponding HI and then oxidizes it to I2. Hence it cannot be used during the reaction of alcohols with KI. Question 7 Name the aromatic salt formed as intermediate during the Sandmeyer's Reaction. Ans. Benzene diazonium halide also known as diazonium salt is formed as intermediate during the Sandmeyer's Reaction. Question 8 Which alkyl halides show colour on exposure to light? Ans. Alkyl Bromide and Alky iodide. Question 9 Give the Swarts reaction for the synthesis of fluoromethane. Ans.

Question 10 Give the order of reactivity of alcohols with a given haloacid. Ans. The order of reactivity of alcohols with a given haloacid is 3°>2°>1°. Chemical properties Question 1 Answer the following: (i) Haloalkanes easily dissolve in organic solvents, why? (ii) What is known as a racemic mixture? Give an example. (iii) Of the two bromo derivatives, C6H5CH(CH3)Br and C6H5CH(C6H5)Br, which one is more reactive in SN1 substitution reaction and why?

Ans. (i) Haloalkanes can easily dissolve in organic solvents of low polarity because the

new forces of attraction set up between haloalkanes and the solvent molecules are of same strength as the forces of attraction being broken.

(ii) A mixture of equal amounts of two enantiomers is known as racemic mixture. For example: When a 3o halide undergoes substitution with KOH, the reaction proceeds through SN1 mechanism forming the racemic mixture in which one of the products has the same configuration as a reactant, while the product has an inverted configuration.

(iii) The SN1 substitution reaction involves the formation of carbocation, which is not

affected by the presence of bulky groups. Thus, C6H5CH(C6H5)Br will be more reactive towards SN1 substitution reaction forming racemic mixture.

Question 2 A solution of KOH hydrolyses CH3 CHClCH2CH3 and CH2CH2CH2CH2Cl.Which one of these is more easily hydrolysed? Ans.

undergoes hydrolysis more easily than .

being a primary alkyl halide has less steric hindrance

than which is secondary alkyl halide.

Question 3

Ans.

Question 4 Give reasons for the following: (i) Grignard reagent should be prepared under anhydrous conditions. (ii) Neo-pentyl bromide undergoes nucleophilic substituting reactions very slowly. (iii) p-Methoxybenzyl bromide reacts faster than p-nitrobenzyl bromide with ethanol to form an ether product. Ans. (i) Grignard reagent reacts with water and gets decomposed, so it is produced in anhydrous conditions.

(ii) Neo-pentyl bromide being a primary halide reacts slowly through SN1, and being a sterically hindered halide reacts slowly even through SN2 mechanism. (iii) Methoxy (-OCH3) group being an electron releasing group stabilizes the intermediate carbocation. On the other hand nitro group (-NO2) is an electron withdrawing group and hence it destabalises the intermediate carbocation. Thus p-methoxy benzyl bromide reacts faster than p-nitrobenzyl bromide because in its case the reaction proceeds via more stable intermediate carbocation. Question 5 A primary alkyl halide (A) C4H9 reacts with alcoholic KOH to give a compound (B). The compound (B) reacts with HBr to give the compound C which is an isomer of A. When A reacts with sodium metal it give a compound (D) whose molecular formula is C8H18. The compound D is different from the compound formed when n-butyl bromide reacts with sodium. Give the structural formula of A and write all the equations involved in the reaction.

Ans. For C4H9Br, two primary halides are possible CH3CH2CH2CH2Br (n-butyl bromide) and (CH3)2CHCH2Br (iso-butyl bromide). Since A is not normal –butyl bromide, it must be iso-butyl bromide.

Question 6 Give reasons for the following: i) Haloalkanes react with KCN to form alkyl cyanides as main product while AgCN forms isocyanide as the chief products. ii) Alcohols do not react with NaBr, but when H2SO4 is added, they form alkyl bromides. Ans. i) KCN is predominantly ionic and provides cyanide ions in solution. Although both carbon and nitrogen atoms of CN can donate electron pairs but the attack takes place mainly through carbon atom and not through nitrogen atom because C-C bond is more stable than C-N bond. However AgCN is mainly covalent in nature and nitrogen is free to donate electron pair forming isocyanide as the main product. ii) Br- is a weak base. So it cannot displace the strong base OH- when H2SO4 is added, it leads to protonation of alcohol, as a result of which water molecule is formed. Since water molecule is a very weak base it is easily replaced by Br-.

Question 7 What is Saytzeff rule? Illustrate with suitable example. Ans. According to Saytzeff rule in dehydrohalogenation reactions the Haloalkane can form more than one alkene due to the availability of more than one alpha hydrogen atoms. In such a case the preferred alkene is that alkene which has greater number of alkyl groups attached to the doubly bonded carbon atoms. For example: the dehydrohalogenation of 2-bromobutane yields two products 1 butene and 2 butene. Out of these 2 butene is the major product (80%) as it is more highly substituted and it is more stable

Question 8 Give the nitration and sulphonation reaction of chlorobenzene. Ans.

Question 9 Which compound in each of the following pairs will react faster in SN2 reaction with -OH? (i) CH3Br or CH3I (ii) (CH3)3CCl or CH3Cl Ans. (i) CH3I will react faster than CH3Br because the order of reactivity is as follows R-I > R-Br > R-Cl > R-F (ii) CH3Cl will react faster than (CH3)3CCl because the order of reactivity is as follows in SN2 reaction. Question 10 What is the directive influence of chlorine in chlorobenzene? Ans. The -Cl group is ortho and para directing in chlorobenzene. Question 11 What is Grignard reagent? Ans. It is an organometallic compound having formula RMgX also called alkyl magnesium halide.

Question 12 Give the order of reactivity for SN1 reaction in methyl, primary, secondary and tertiary halide. Ans. Methyl halide >Primary halide > secondary halide> Tertiary halide

Question 13 What are ambident nucleophiles? Ans. Nucleophiles that possess two nucleophilic centres are called ambident nucleophiles. Example cyanide

Polyhalogen compounds uestion 1

(i) State one use each of DDT and iodoform.

(ii) Which compound in the following couples will react faster in SN2

displacement and why?

(a) 1-Bromopentane or 2-bromopentane

(b) 1-Bromo-2-methylbutane or 2-bromo-2methylbutane.

Ans. (i) Use of DDT: It is used as an insecticide. Use of iodoform: It is used as an antiseptic. (ii) (a) 1 – Bromopentane will undergo faster SN2 displacement reaction than 2-bromopentane because 1- bromopentane has less steric hindrance than 2 –bromopentane. This is because 1- bromopentane is a primary alkyl halide whereas 2-bromopentane is a secondary alkyl halide. (b) 1- Bromo-2-methylbutane will undergo SN2 reaction faster than 2- Bromo-2-methylbutane because 1- Bromo-2-methylbutane has less steric hindrance than 2- Bromo-2-methylbutane. This is because 1- Bromo-2-methylbutane is a primary alkyl halide whereas 2- Bromo-2-methylbutane is a tertiary alkyl halide.

Question 2 What is DDT and why its use is banned in United States? Ans. DDT is p, p'-Dichlorodiphenyltrichloroethane and is used as insecticide. It is banned for the following reason (i) Its high toxicity towards fish. (ii) DDT is not metabolized very rapidly by animals but get deposited and stored in the fatty tissues. Question 3 Give the three uses of each of the following: (i) Dichloromethane (ii) Tetrachloromethane Ans. Dichloromethane: It can be used (i) As solvent (ii) As paint remover (iii) As aerosol propellant Tetrachloro methane: It can be used (i) As a solvent (ii) As a fire extinguisher (iii) As a degreasing agent Question 4 What is the reason that Haloarenes are less reactive than haloalkane towards nucleophilic substitution reaction? Ans. In haloalkane, the halogen is attached to sp2 hybridised carbon atom while in case of haloarene it is attached to sp2 hybridised carbon atom. The sp2 hybridized carbon have greater s-character than sp3-hybridised carbon and thus more electronegative and can hold the electron pair of C—X bond more tightly than in haloalkane . The, C—Cl bond length in haloalkane is greater than in haloarene. Since it is difficult to break a shorter bond than a longer bond, therefore, Haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction. Question 5 Why chloroform is kept closed in dark colored bottles? Ans. Chloroform is oxidized slowly by air in the presence of light to an extremely poisonous gas, carbonyl chloride, also known as Phosgene

Hence it is kept closed in dark colored bottles. Question 6 Give the role of carbon tetrachloride and Freon on environmental degradation. Ans. Both the carbon tetrachloride and Freon causes depletion of ozone layer when released in air because they remain unchanged and diffuses into stratosphere. The depletion of ozone layer increases the exposure of UV rays to human being which results in skin cancer, eye disorder and immune system diseases. Question 7 Why the use of chloroform as anesthetic is decreasing? Ans. Inhaling chloroform vapors depresses the central nervous system and its chronic exposure may cause damage to the liver and kidneys due to metabolism of chloroform to phosgene gas. Hence the use of chloroform as anesthetic is decreasing. Question 8 Give two hazardous effects of methylene dichloride. Ans.

i) It harms the central nervous system. (ii) On direct contact with skin causes severe burning.

Question 9 What is iodoform? Give one use of it. Ans. Triodomethane is called iodoform and can be used as antiseptic due to liberation of free iodine. Question 10 What are freons? Give one example. Ans. Freons are chloro fluoro carbons used as refrigerants. Example CCl2F2. Give the full name of DDT. Ans. p,p’-Dichlorodiphenyltrichloroethane @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@